As hard disk drive apparatuses increase in capacity and reduce in size, highly sensitive and high-resolution thin-film magnetic heads are being demanded. In order to satisfy the demand, giant magentoresistive effect (GMR) thin-film magnetic heads with GMR read head elements each having a multi-layered structure with a magnetization-fixed layer and a magnetization-free layer become widely used. On the other hand, tunnel magnetoresistive effect (TMR) thin-film magnetic heads with TMR read head elements having higher sensitivity and higher resolution are put to practical use.
In thin-film magnetic heads with MR read head element, defective products that generate noises, for example Barkhausen noise, in their outputs may be included. The Barkhausen noise is generated primarily because a magnetic domain wall is caught in defects in the magnetic film constituting the MR read head element as the magnetic domain wall moves, and is affected substantially by stress applied to the MR read head element. As the noise generated, the performance of the magnetic head is weakened, for example, the flying height of the magnetic head is unstable and hard to control, which cause the stability is reduced and, in turn, the reading performance is weakened.
Thus, a noise testing process must be carried out before the magnetic head product is put into use. One of common methods for determining whether a magnetic head is acceptable or defective by measuring noise generated in response to sense current with various frequency bandwidths and various values flowing through the magnetic head. And noise profile can be obtained and a noise level of the magnetic head can be measured out, in turn, an acceptable or defective magnetic head can be estimated and judged. Concretely, this conventional noise testing method includes two types which are quasi-static testing and dynamic testing.
Accordingly, a quasi-static testing machine and a dynamic testing machine are used. The quasi-static testing machine is a testing machine that saves time, and the efficiency is higher, which can measure the noise profile of the magnetic head with 0˜80 MHz however. That is, the current limited frequency bandwidth can be extended is 80 MHz merely. It's very difficult to measure out the noises generated in higher frequency bandwidth over 80 MHz. Therefore, the dynamic testing machine is used for detecting the noise under a wider frequency bandwidth extended. Currently, the frequency bandwidth of the dynamic testing machine can be up to very high, such as 1 GHz, even higher which depends on the testing unit. Commonly, the relationship between the frequency bandwidth and the noise level can be represented by a noise profile, which illustrates a noise feature of the magnetic head. From the noise profile, the noise level can be measured out. Compared with the quasi-static testing method, the dynamic testing method has a higher accuracy, the noise happened in the higher frequency bandwidth also can be detected. However, the dynamic testing method takes a long time, and the testing efficiency is quite low. Thus it would be unpractical if branches of magnetic heads need to be tested.
Thus, the people are eager for an improved quasi-static testing method to perform a noise testing to the magnetic head under a wider frequency bandwidth range by using a quasi-static testing machine, which can save time, improve testing efficiency and achieve high accuracy.